With the use of an ignition magneto, it is desirable to obtain maximum output in a minimum period of time and to maintain the maximum output throughout the entire speed range. At low speeds, where little energy is generated, the magneto must be highly efficient. The efficiency of the magneto is dependent in large part on the proximity of the primary winding to the core.
In the past, the primary winding has been hand wound and subsequently taped to hold the windings in place. The primary winding is positioned around an upstanding chimney that extends upwardly from the bottom surface of a cup-shaped plastic casing, and after installation of the secondary winding, the unit is potted in a thermosetting resin. The hand wound or air core winding has the advantage that the primary winding is in close proximity to the iron core, separated only by the thickness of the plastic chimney. However, the hand wound core is extremely expensive from a labor standpoint.
Another manner of producing the primary winding has been to wind the wire on a plastic bobbin, with the free ends of the wire being held in grooves in the end flange of the bobbin. The bobbin is then snap-fitted within an opening in the bottom surface of the casing, and after installation of the secondary winding, the unit is potted. With this method, the bobbin replaces the chimney in the air core method, so that the primary winding is in close proximity to the iron core, being separated only by the wall thickness of the bobbin.
Recently, the transistor and trigger coil have been formed integrally with the outer casing of the ignition magneto. With this type of construction, the transistor and trigger coil are mounted within a depression in the bottom surface of the casing and are potted along with the primary and secondary windings. However, with the integral transistor and trigger core, the snap-in type of bobbin cannot be used, because of a lack of space.
With the use of the integral transistor and trigger coil, the core for the trigger coil has been traditionally inserted after potting. This requires a hole to be drilled through the casing in precise alignment with the opening in the trigger coil for insertion of the core. It has been found that the installation of the trigger core is a difficult and time consuming operation, because of the required precise alignment and minimum tolerances that are involved.